Selection of Tool Steel for Trimming Dies
Trimming dies are secondary-operation tools used to remove excess material—such as flash, gates, overflows, and vents—from forged or die-cast parts. Depending on the process, trimming may be performed cold (typically below 150 °C / 300 °F) or hot (up to 980 °C / 1800 °F for certain alloys).
During operation, the cutting edge is subjected to high shear stress, impact loading, and continuous friction. In forging applications, oxide scale further accelerates abrasive wear. As a result, trimming die failure is most often driven by edge chipping, fracture, or rapid edge degradation rather than uniform wear.
Selection Factors
Tool steel selection for trimming dies is governed by the balance between wear resistance and toughness, as well as thermal stability during hot operations.
Wear Resistance vs. Toughness
The cutting edge must resist abrasive wear from scale and workpiece material to maintain sharpness and dimensional accuracy.
However, higher hardness and carbide content reduce toughness. In trimming operations—especially with thick sections, uneven loading, or misalignment—impact loads are unavoidable. If toughness is insufficient, failure occurs as edge chipping or brittle fracture rather than gradual wear.
The requirement is not maximum hardness, but a balance that maintains edge stability under both abrasion and impact.
Hot Hardness and Heat Checking Resistance
In hot trimming, thermal effects become critical.
The tool steel must retain strength at elevated temperatures to prevent edge deformation. At the same time, repeated heating and cooling cycles generate thermal fatigue, leading to surface cracking (heat checking).
Once cracks initiate, they propagate under mechanical stress, accelerating edge failure. Therefore, resistance to thermal softening and thermal fatigue directly determines tool life in hot trimming conditions.
Recommended Tool Steels
AISI D2 Tool Steel |1.2379 | SKD11
AISI D2 is widely used where abrasive wear is the dominant factor, particularly in cold trimming operations with long production runs.
Its high carbon and chromium content produce a large volume of hard carbides, providing strong resistance to abrasion and excellent edge retention. Typically used at 58–60 HRC, D2 maintains cutting performance over extended cycles.
Due to its limited toughness, D2 is not suitable for heavy impact conditions. It may be used in restricted hot trimming applications (such as stainless steels) where wear resistance is more critical than thermal fatigue resistance, but it is not a general hot-work solution.
AISI A2 Tool Steel | 1.2363 | SKD12
AISI A2 provides a more balanced combination of wear resistance and toughness.
With lower carbide content than D2, A2 offers improved resistance to edge chipping while maintaining adequate wear performance. It is suitable for cold trimming operations where loading conditions are less stable, or impact cannot be avoided.
At 56–58 HRC, A2 is commonly selected when D2 exhibits premature edge failure, but higher wear resistance than that of shock steels is still required.
AISI S7 Tool Steel | 1.2355
AISI S7 is designed for applications dominated by impact and shock loading.
Its high toughness allows it to resist cracking and edge failure under severe mechanical stress, making it suitable for heavy-gage trimming and interrupted cuts.
At 56–58 HRC, S7 performs reliably in unstable conditions. The trade-off is lower wear resistance, so it is not suitable for long-run applications where abrasion is the primary failure mechanism.
H13 Tool Steel | 1.2344 | SKD61
AISI H13 is the standard material for hot trimming operations.
It provides a stable combination of toughness, hot strength, and resistance to thermal fatigue. Unlike cold-work steels, H13 maintains its mechanical properties under repeated thermal cycling.
Typically used at 44–52 HRC, H13 resists heat checking and slows crack propagation, making it suitable for most die-casting and hot-forging trimming applications.
AISI H21 (Tungsten Hot-Work Tool Steel)
AISI H21 is selected for extremely high-temperature conditions.
Its high tungsten content provides superior hot hardness and resistance to softening, allowing it to retain strength at temperatures up to approximately 620 °C (1150 °F).
At 50–52 HRC, H21 is suitable for trimming heavy flash in upset forging and other high-temperature, high-load applications where H13 may soften too quickly.
Summary Table
| Tool Steel Grade | Key Properties / Hardness | Main Advantage for Trimming Dies |
| AISI D2 | 58–60 HRC; ~1.5% C, 12% Cr | High abrasion resistance for long-run cold trimming |
| AISI A2 | 56–58 HRC; 1.00% C, 5.00% Cr | Balanced wear resistance and toughness |
| AISI S7 | 56–58 HRC; Shock-resisting | Maximum resistance to impact and edge chipping |
| AISI H13 | 44–52 HRC; Chromium hot-work | Resistance to heat checking and thermal fatigue |
| AISI H21 | 50–52 HRC; Tungsten hot-work | High hot hardness for extreme-temperature trimming |